Communications of the ACM

practice

MOST DEVELOPERS UNDERSTAND that reading uninitialized variables in C is a defect, but some do it anyway—for example, to create entropy. What happens when you read uninitialized objects is unsettled in the current version of the C standard (C11). 3 Various proposals have been made to resolve these issues in the planned C2X revision of the standard. Consequently, this is a good time to understand existing behaviors as well as proposed revisions to the standard to influence the evolution of the C language. Given the behavior of uninitialized reads is unsettled in C11, prudence dictates eliminating uninitialized reads from your code.

This article describes object initialization,
indeterminate values, and trap representations and
then examines sample programs that illustrate the
effects of these concepts on program behavior.

Initialization

Understanding how and when an object is initialized is necessary to understand the behavior of reading an uninitialized object.

An object whose identifier is declared with no linkage (a file scope object has internal linkage by default) and without the storage-class specifier static has automatic storage duration. The initial value of the object is indeterminate. If an initialization is specified for the object, it is performed each time the declaration or compound literal is reached in the execution of the block; otherwise, the value becomes indeterminate each time the declaration is reached.

Subsection 6. 7. 9 paragraph 10 of
the C11 Standard4 describes how ob-
jects having static or thread storage du-
ration are initialized:
If an object that has automatic stor-
age duration is not initialized explicitly,
its value is indeterminate. If an object
that has static or thread storage dura-
tion is not initialized explicitly, then:

˲ if it has pointer type, it is initialized to a null pointer;

˲ if it has arithmetic type, it is initialized to (positive or unsigned) zero;

˲ if it is an aggregate, every member is initialized (recursively) according to these rules, and any padding is initialized to zero bits;

˲ if it is a union, the first named member is initialized (recursively) according to these rules, and any padding is initialized to zero bits.

Many of the dynamic allocation functions do not initialize memory. For example, the malloc function allocates space for an object whose size is specified by its argument and whose value is indeterminate. For the real-loc function, any bytes in the new object beyond the size of the old object have indeterminate values.

Indeterminate Values

In all cases, an uninitialized object has an indeterminate value. The C standard states that an indeterminate value can be either an unspecified value or